1. Field of the Invention
The present invention relates to a novel lubricating oil composition and to a method for supplying the lubricating oil composition.
2. Description of the Prior Art
Among conventional rolling oils which are generally used as lubricants for cold rolling of steel slabs, plates or the like, there are animal- or plant-type oils and mineral-type oils. As a lubricating oil component, the former type oils contain triglyceride, while the latter type oils contain a petroleum hydrocarbon into which an oilness improver, extreme pressure agent, antioxidant, etc. are formulated. Such conventional oils are employed together with an emulsifier as 1-20% oil-in-water (O/W) type emulsions.
However, such emulsion-type lubricating oil agents encounter various drawbacks and are not satisfactory.
Many studies have been made on lubricating oil compositions to solve the drawbacks of the conventional lubricating oil compositions. As a result of these studies, it has been found that a composition comprising a liquid lubricating oil component, fatty acid or its glyceride, and wax such as slacks wax is favorable since it serves as a protective coating agent for the surfaces of stored metallic articles and effectively allows any contaminants, which would occur in such emulsion, to float thereon (Japanese Patent Publication No. 42927/1973). Another finding is that a lubricating oil composition containing paraffin wax and an oxidation product of an .alpha.-olefin exhibits good lubricating properties and causes almost no oil stains (Japanese Patent Publication No. 7174/1978, and Japanese Laid-open Patent Application Nos. 67906/1974 and 82707/1974). Notwithstanding these efforts, such lubricating oil compositions still remain unsatisfactory.
A rolling oil takes an important role in the lubrication of the arcuate contact area between a roll and a steel strip and prevents the roll and steel strip from being brought into direct contact with each other under high load conditions. In the cold rolling techniques, there has been a marked tendency toward high speed rolling to increase productivity as well as toward omission of a surface cleaning step such as electrolytic cleaning to simplify any treatment steps after rolling. Thus, a need continues to exist for the development of a rolling oil which can withstand high load and high speed rolling and which requires no cleaning step, and further research to this end is presently being conducted.
Although it is important to select any suitable lubricating oil component, oilness improver, extreme pressure agent, etc., by which a rolling oil is prepared, in order to improve the quality of the rolling oil, particular importance lies in controlling the nature of a system in which a lubricating oil composition is emulsified in water for actual use. In other words, any conventional rolling oil is supplied in the form of an aqueous emulsion for rolling lubrication operation. Even if there is no difference in the compositions of the lubricating oil components, the amount of the oil which adheres to the surfaces of a roll and a steel strip (i.e., the plated-out quantity) would vary depending on the stability of the emulsified particles. Therefore, the quantity of the rolling oil to be taken into the arcuate contact area between the roll and the steel strip would change from one lubricating oil composition to another and result in varied rolling lubrication characteristics.
Where a great deal of an oil is rolled into the arcuate contact area between a roll and a steel strip, lubrication would generally be improved on the surface of the strip, on which surface the size of the steel strip is being expanded by virtue of plastic deformation. This renders small the contact area between the roll and the strip and hence improves the rolling lubrication conditions.
Therefore, where a rolling oil is used in the form of an aqueous dispersion, it is necessary to prepare the dispersion system as unstably as possible to increase the plated-out quantity. On the other hand, where a rolling oil is formulated into an aqueous composition and where such aqueous composition is circulated, it is preferred that the aqueous dispersion system be stable and easy to handle.
In general, a lubricating oil agent is continuously circulated over a long period of time during which it is susceptible of being tainted by, besides scum and dirty oil, so-called "contaminants" such as fine metal powders, a lubricating oil for roll bearings, an anti-rusting oil applied after pickling and the like. When such contaminants mix into the rolling oil and adhere to the surfaces of a roll and a steel strip, they have adverse effect on the rolling lubrication characteristics and deteriorate the surface cleanliness of a steel plate obtained by the rolling operation. If the steel plate is subjected to an annealing step without any pretreatment, oil stains may occur on the surface of the steel plate due to the adherent oil or contaminants. Thus, it is desired that such contaminants not mix into the lubricating oil agent during the circulation of the agent but separate from the agent and float into the top layer of the agent or precipitate into the bottom layer for easy removal of the contaminants from the rolling oil.
As described above, a rolling oil has hithertofore been emulsified in water together with an emulsifier to form an aqueous emulsion. The stability of the emulsion (E.S.I.) is adjusted by controlling the content of the rolling oil and the HLB value (normally 8 to 14). In such emulsion-type rolling oil which is prepared by emulsifying a lubricating oil component in water, the plated-out quantity tends to be in inverse proportion to the E.S.I. If the stability of the emulsion is increased, the plated-out quantity relative to a steel plate becomes decreased, thereby rendering the lubrication insufficient. On the other hand, if the plated-out quantity is increased, the emulsion becomes unstable and creates various obstacles to circulated application of the rolling oil.
Moreover, the emulsion-type rolling oil involves scum or floating oil fractions due to polymerization or decomposition of the emulsion during its circulation. Lowering the concentration of the emulsion adversely affects its lubrication properties, thereby causing accidents such as burning or damage to expensive rolls, and further developing heat marks on rolled steel plates and spoiling the quality of the products. Furthermore, the lubricating oil agent per se is tainted by the absorption of the above-described scum, floating oil fractions, fine metal powders, lubricating oil for roll bearings, anti-rusting oil, etc. Such contaminants are taken into the emulsion by the action of the emulsifier, but it is difficult to separate and remove the contaminants. During the circulation of the rolling oil, the content of such contaminants becomes higher such that it is impossible to avoid re-adhesion of the contaminants to the surface of a steel plate during rolling operation.
In view of the situation with the existing techniques, the present inventors have made extensive studies to surmount the above-noted drawbacks of the conventional rolling oils and have found that extremely good characteristics can be achieved by dispersing a lubricating oil component in water by using some specific water-soluble dispersants.
More particularly, where a lubricating oil component is an oil, fat or wax having a melting point in the range of 20.degree. to 100.degree. C., the lubricating oil component is stably dispersed in water in a solid state at a temperature not higher than its melting point. When supplied to a machined portion at a temperature of at least its melting point, the lubricating oil component becomes unstable and adheres to the machined portion, thereby exhibiting good lubrication action.
If a lubricating oil component is an oil or fat of a melting point lower than 20.degree. C., the oil particles are dispersed in water in a relatively large particle size, as is different from the conventional emulsions. Accordingly, such lubricating oil component shows good plating-out properties on rolls and steel plates having surfaces of high energy during rolling operation. In addition, since agglomeration of the oil particles is inhibited by the action of the dispersant, the dispersion is kept stable. As compared with the conventional emulsions, less contaminants are liable to mix into the present dispersion. Even if mixed, the contaminants may be removed easily.